1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) apparatus and its control method, utilizing fast spin echo (FSE) method.
2. Description of the Related Art
In magnetic resonance imaging apparatuses of this type, phase lags of spin echoes are detected by pre-scanning. In actual scanning, the detected phase lags are corrected.
For instance, U.S. Pat. No. 6,369,568 discloses a technique, in which spin echoes without phase encodes (PE) are collected, and the 0th- and 1st-order phase differences between the 1st and 2nd echoes are measured. The 0th-order phase difference is mainly corrected using an RF phase, and the 1st-order phase difference is corrected by adding a correction pulse to a gradient magnetic field pulse in the readout direction.
The level of the correction pulse is theoretically determined utilizing, for example, a substantially trapezoidal model. However, a change in gradient magnetic field may vary because of the influence of eddy currents or vibration, even if a preset correction pulse is added. In particular, a gradient magnetic coil of a non-shield type and/or open-type magnet is significantly influenced by eddy currents or vibration, and considerable variations are found in the area of the gradient magnetic field.
Accordingly, when a correction pulse uniquely determined from a measured phase difference is used, the phase difference may not completely be corrected. The remaining phase difference may cause variations in reception level.